title: Green and black tea extracts inhibit HMG-CoA reductase and activate AMP kinase to decrease cholesterol synthesis in hepatoma cells.
abstract: Recent studies have demonstrated that green and black tea consumption can lower serum cholesterol in animals and in man, and suppression of hepatic cholesterol synthesis is suggested to contribute to this effect. To evaluate this hypothesis, we measured cholesterol synthesis in cultured rat hepatoma cells in the presence of green and black tea extracts and selected components. Green and black tea decreased cholesterol synthesis by up to 55% and 78%, respectively, as measured by a 3-h incorporation of radiolabeled acetate. Inhibition was much less evident when radiolabeled mevalonate was used, suggesting that the inhibition was mediated largely at or above the level of HMG-CoA reductase. Both extracts directly inhibited HMG-CoA reductase when added to microsomal preparations, although the extent of inhibition was considerably less than the decrease in cholesterol synthesis observed in whole cells. As HMG-CoA reductase activity also can be decreased by enzyme phosphorylation by AMP kinase, the phosphorylation state of HMG-CoA reductase and AMP kinase, which is activated by phosphorylation, was determined in lysates from cells treated with tea extracts. Both extracts increased AMP-kinase phosphorylation and HMG-CoA reductase phosphorylation by 2.5- to 4-fold, but with different time courses: maximal phosphorylation with green tea was evident within 30 min of treatment, whereas with black tea phosphorylation was slower to develop, with maximal phosphorylation occurring > or =3 hours after treatment. These results suggest that both green and black tea decrease cholesterol synthesis in whole cells by directly inhibiting HMG-CoA reductase and by promoting its inactivation by AMP kinase.
authors.affiliation: Department of Pharmaceutical Sciences, College of Pharmacy, University of Kentucky, Lexington, KY 40536, USA.
authors.forename: Dev K
authors.initials: DK
authors.lastname: Singh
authors.forename: Subhashis
authors.initials: S
authors.lastname: Banerjee
authors.forename: Todd D
authors.initials: TD
authors.lastname: Porter
mesh_terms: D000263:Adenylate Kinase / Q000378:metabolism*; D000818:Animals; D045744:Cell Line, Tumor; D002784:Cholesterol / Q000096:biosynthesis*; D003470:Culture Media; D004789:Enzyme Activation; D006903:Hydroxymethylglutaryl CoA Reductases / Q000378:metabolism; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors / Q000494:pharmacology*; D008114:Liver Neoplasms, Experimental / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D010936:Plant Extracts / Q000494:pharmacology*; D051381:Rats; D013662:Tea / Q000737:chemistry*
chemical_list: D003470:Culture Media; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors; D010936:Plant Extracts; D013662:Tea; D002784:Cholesterol; D006903:Hydroxymethylglutaryl CoA Reductases; D000263:Adenylate Kinase